1. Field of the Invention
The present invention is related to a method to determine target azimuth by the use of an ASR (Airport Surveillance Radar) type radar. It can be applied in particular to surveillance radars used, for example, for air traffic control applications.
A primary radar has the talks not only of indicating the presence of targets of interest within its detection zone, but also of providing data on these targets, especially their position in terms of distance and azimuth. The primary radars used, for example, for air traffic control, are essentially classic 2D radars. For reasons of economy in particular, they do not use single-pulse type techniques to prepare azimuth data. Thus, azimuth data is usually obtained by barycenter type computation methods in which the azimuth values of the different detections that form the radar blip are weighted by their amplitude.
2. Description of the Prior Art
In earlier radars, there used to be a large number of detection information elements pertaining to a target per period of illumination of the target, since these radars did not use Doppler processing. For example, a classic illumination time of 15 ms for a precision approach radar emitting a recurrence signal approximately every 1 ms gave 15 recurrence zones and hence 15 detections.
The Doppler processing radars there were subsequently used carried out a sliding filter operation on the n last recurrences so that a non-negligible quantity of data remained.
Then, the techniques of MTD (mobile target detector) Doppler filtering techniques combined with burst mode operation used to improve the detection of targets in the presence of ground or rain clutter resulted in a drastic reduction of the number of detections on each target. In very many cases, there were only one or two detections on distant targets or targets with low radar equivalent surface. To give an order of magnitude, and referring to the previous example, the detection rate went from 15 pieces of information on position (i.e. the number of valid information elements needed to extract the azimuth position) per burst to two.
The emergence of instantaneous wideband solid-state transmitters that enable working in two frequencies to improve the detection by diversity effect has the additional consequence of further halving the number of detections on each frequency.
At fixed frequencies, with two pieces of detection information, it is still possible to obtain an extraction of azimuth position. With the use of two frequencies, since the number of detections is halved, there is only one detection per target. Thus, the azimuth of the target can only be associated with the azimuth of the detection. It can then be assumed, in an initial approximation, that the error is distributed on more or less one half-length of the antenna lobe. The position error thus becomes great.
One aim of the invention especially is to improve the azimuth precision of a radar, for example of the ASR type. To this end, an object of the invention is a method to determine the azimuth of a target by means of a radar that sends out a wave of detections by N-pulse bursts. The method comprises at least the following steps in one burst:
a step in which the burst is split up into two half bursts, one front burst and one rear burst;
a step for filtering the half burst to associate an azimuth value xcex81 and an amplitude value A1 with the front burst, and an azimuth value xcex82 and an amplitude value A2 with the rear burst;
a step to obtain the azimuth Az of the target at a value corresponding to the maximum of a Gaussian curve passing through the points having components (xcex81, A1), (xcex82, A2).
Preferably, the front and rear half bursts have the same number of pulses, N/2.
The main advantages of the invention are that it substantially improves performance in discriminating between two targets located at the same distance and separated by a small distance at azimuth, does not complicate the radar processing and is simple to implement.